1. Field of the invention
This invention generally relates to semiconductor devices, and more particularly, to semiconductor devices having bump terminal electrodes to which metallic connecting leads are bonded.
2. Description of the prior art
Various new methods have been proposed to make electrical connection of connecting leads to the metallic terminal electrodes of semiconductor devices in place of the conventional wire bonding. One technique that has been recently proposed for LSI application is gang bonding in which connecting leads are prepared from a stripe of metal foil on a plastic tape. The end portions of the leads the thinly formed to be directly and simultaneously bonded to the metallic protrusions (bumps) at the IC terminal electrodes. A gang bonding method of this type is described, for example, in U.S. Pat. No. 3,763,404, U.S. Pat. No. 4,051,508, and "SOLID STATE TECHNOLOGY" October, 1975, pp 46-52.
The bumps are usually formed of gold and are located at the edge of a wiring layer extending on the insulating film of the semiconductor substrate and are connected to a region thereof. The bump is about 100 .mu.m wide at each side and about 10.about.20 .mu.m high from the upper surface of the semiconductor substrate. The leads that are to be connected to the bumps consist of tin- or gold-plated copper foils, which have a thickness of 20.about.40 .mu.m and a width of about 100 .mu.m, which is substantially equal to the bump size.
The connection method by tin-plated copper leads is widely used because a gold-tin eutectic can be obtained at a low temperature and under a low bonding pressure. The use of leads of this type, however, present a critical problem wherein the single crystalline tin whisker causes an electric short between the leads. For this reason, gold-plated copper leads are employed in a high reliability device since they show no whisker formation. However, when a gold-plated copper lead is bonded to a gold bump, a greater bonding load is required to obtain satisfactory mechanical strength at the connection between the lead and the bump in comparison with tin-plated copper leads. As a result, both lead and bump cause plastic deformation due to the heat and load applied thereto at the time of bonding and the gap between the lead and the upper surface of the semiconductor element becomes smaller than the height of the bump. Moreover, the size of a bonding tool is generally larger than the size of a square defined by a plurality of bumps in order to facilitate the accurate positioning of the tool with respect to the bumps. This means that the lead is also applied with a load at the external portion of the bump and bent towards the upper surface of the semiconductor element.
Generally, a plurality of semiconductor elements are formed on a single semiconductor wafer. After the electric characteristics of these elements are checked by applying a test probe to the bumps, they are separated into the individual semiconductor elements by scribing with a tapered diamond or a laser beam or by cutting the wafer with a thin diamond wheel. The upper surface of the semiconductor element is covered with an insulating film, e.g. a silicon oxide film, but at the side faces of the element, the semiconductor substrate is exposed. Since the surface-insulating film is generally made of a brittle material, the contour portions of the side faces and the upper surface are chipped off and the semiconductor substrate is also frequently exposed there.
On the other hand, the bonding surface of the copper foil as a base member of the leads, that is, the surface to be attached to an insulating film, is coarsened. Accordingly, when an electrolytic plating of gold is applied to the bonding surface, the unevenness of the bonding surface is further enhanced due to the electrolytic density.
If this lead and semiconductor device are connected by gang bonding, a critical drawback of an electric short-circuit (hereinafter referred to as "edge-short") tends to occur between the bottom surface of the leads and the exposed silicon at the contour portion of the semiconductor substrate because the leads are bent towards the upper surface of the substrate as described previously. Even if isolation is maintained by a slight gap between the leads and the surface of the semiconductor element, such a semiconductor device involves a critical drawback with respect to its reliability in that when the device is incorporated in an apparatus, the edge-short may still occur due to the thermal expansion of the leads, or the contamination of the leads by dust or the like.
In order to solve this problem, another method has been prepared in which bonding is effected after the leads are formed in such a shape as to expand the gap at the edge portion of the semiconductor substrate. However, this plastic forming method significantly lowers the utilization rate of the lead frame because the positions of the tips of the leads are deviated during the formation of the leads especially when a number of leads are used, and a back-spring phenomenon of the copper member occurs. In addition, the method is not an effective counter-measure for the abovementioned bending phenomenon of the leads towards the surface of the semiconductor elements during bonding. For these reasons, this method cannot provide a solution for the edge-short. In yet another method that has been prepared the leads are shaped after they are bonded to the bumps in order to enlarge the gaps between the leads and the contour portions of the semiconductor substrate. However, this method again presents a new problem in that lowering of the strength of the lead itself and strength of the bond are inevitable because the method extends the leads per se. Furthermore, the once shaped leads themselves are again bent during post-treatment or by handling. Hence, this shaping method is also not an effective solution to the problem.